1. Field of the Invention
The invention relates to the fabrication of semiconductor laser devices, and in particular the use of molecular beam epitaxial techniques to produce sequential layers of strained crystalline material for use in laser devices having improved threshold current density characteristics.
2. Description of the Prior Art
The use of strained layers in semiconductor devices is shown in the prior art for a limited number of applications. In U.S. Pat. No. 4,376,138, a waveguiding layer is created by an n-type InP epitaxial layer on an n.sup.+ -InP substrate. The lateral confinement of the radiation is achieved through the strain induced in the epitaxial layer by a thick metal film which cools upon evaporation. Strain in a semiconductor layer is analogous to the deformation of an elastic body under mechanical stretching action. In a semiconductor body, a lattice mismatch between adjacent epitaxial layers results in a displacement of atoms in the crystal structure which may be defined as a strain. Such displacement of atoms in a plane parallel to the major surface of the layer typically results in modified electrical properties of the strained layer.
In U.S. Pat. No. 4,445,965 a method for making thin film cadmium telluride and related semiconductors for solar cells is described. In such a technique, a semiconductor sandwich is attached to a rigid supporting substrate such as glass by a suitable adhesive and strain is applied so that the thin telluride layer fractures and the desired thin film, cadmium telluride layer is obtained attached to a supporting glass substrate.
In the article "Lasing Transitions in GaAs/GaAs.sub.1-x P.sub.x Strained-Layer Superlattices with x=0.1-0.5", P. L. Gourley, J. P. Hohimer, and R. M. Biefeld, Appl. Phys. Lett. Vol. 47(6) Sept. 15, 1985, pp. 552-554, the role of strain in modifying lasing transition energy and gain coefficient in photopumped GaAs/GaAs.sub.1-x P.sub.x strained-layer superlattices is investigated by examining photoluminescene, excitation, and lasing spectra for sample with x in the range 0.1-0.5. However, such prior art is directed to devices having an increase of laser gain rather than a reduction of laser threshold current density. Furthermore, the predicted increase in gain, in the prior art is for an electric polarization orthoganal to the polarization in which the threshold current is actually reduced. Moreover, the amount of strain used in such prior art is, in fact, insufficient to appreciably affect the current density. In addition, the devices described in such prior art use a complex multiple layer superlattice structure rather than a single active layer. In summary, prior to the present invention, there has not been a semiconductor laser device structure in which strain introduced in a layer of the device has reduced the lase threshold current density of the laser.